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(emacs-lisp-intro.info)Change a defun

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Change a Function Definition
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   If you want to change the code in `multiply-by-seven', just rewrite
it.  To install the new version in place of the old one, evaluate the
function definition again.  This is how you modify code in Emacs.  It is
very simple.

   As an example, you can change the `multiply-by-seven' function to
add the number to itself seven times instead of multiplying the number
by seven.  It produces the same answer, but by a different path.  At
the same time, we will add a comment to the code; a comment is text
that the Lisp interpreter ignores, but that a human reader may find
useful or enlightening.  The comment is that this is the "second
version".

     (defun multiply-by-seven (number)       ; Second version.
       "Multiply NUMBER by seven."
       (+ number number number number number number number))

   The comment follows a semicolon, `;'.  In Lisp, everything on a line
that follows a semicolon is a comment.  The end of the line is the end
of the comment.  To stretch a comment over two or more lines, begin
each line with a semicolon.

   Note: Beginning a `.emacs' File.emacs File, and Note:
Comments, for more about comments.

   You can install this version of the `multiply-by-seven' function by
evaluating it in the same way you evaluated the first function: place
the cursor after the last parenthesis and type `C-x C-e'.

   In summary, this is how you write code in Emacs Lisp: you write a
function; install it; test it; and then make fixes or enhancements and
install it again.